The Southampton plant, which employed 500 workers, was Ford's last vehicle assembly plant in Britain. Production has fallen from 66,000 in 2008 to 28,000 last year, when the work was reduced to a single shift.
Ford now has 11,500 workers at plants in Britain, the Unite union said.
Ford promised further investments at the Dagenham plant in east London to support production of a new 20-litre, four-cylinder low-CO2 diesel engine family, and at the Bridgend plant in Wales to support gasoline engine manufacturing.
"Investing in what we do best here in the U.K., and building on our strengths is the only way to deal decisively with the new economic reality in Europe, and help build a profitable business," said Joe Greenwell, chairman of Ford of Britain.
Len McCluskey, general secretary of the Unite union, said workers would fight against the closures.
"This announcement has been handled disgracefully," McCluskey said "Only a few months ago Ford was promising staff a new transit model for Southampton in 2014."
Closing the Belgian and British plants reduces vehicle assembly capacity, excluding Russia, by 18 percent, or 355,000 units, yielding annual savings of at least $450 million, Ford said.
Ford's sales were down 14.9 percent in September compared with a year ago, worse than the 10.8 percent fall for all European vehicle brands, according to Acea, the European carmakers' association.